The main domain or to a subdomain of the main domain.You can have one wordpress.com blog mapped to the main domain and as many others as you want mapped to as many subdomains as you want. Normally that is done by setting the CNAME at your registrar,e.g. CNAME blogname.domain.com pointing to blogname.wordpress.com.That would make blogname.wordpress.com the blogname subdomain of domain.com.Whether you can access your mail at mail.domain.com or not has nothing to do with the subdomain setup at wordpress.com. You need a mail hosting service to do that.